This invention relates to radio communication. In particular, this invention relates to the use of a same-frequency repeater.
Radio communication between one transceiver and another is frequently accomplished by having both transceivers transmit on a first frequency and receive on another. A special transceiver called a primary repeater receives the transmissions on the first frequency and rebroadcasts them on the second frequency. The cost in bandwidth for such a system is the bandwidth allotted to two transmitted signals. When it is desired to extend the area of coverage beyond that served by the primary repeater, either to fill shadows in coverage or to extend boundaries of coverage beyond the area readily reachable by the primary repeater, it is desirable to use one or more secondary repeaters. Typically, when broadcasting signals that are not directional, the secondary repeater transmits on a third frequency and receives on a fourth. The mobile units receive on the third frequency and transmit on the fourth. Since, in this arrangement, two systems are operating simultaneously on different frequencies, the cost in bandwidth for such a system is twice that for the single-channel system.
When it is desired to have a mobile unit receive transmissions directly from the primary repeater as well as from a secondary repeater, or when it is desired to have overlapping coverage from a plurality of repeaters, then a same-frequency repeater (SFR) is useful. An SFR is a transceiver that receives a transmission on a particular frequency and rebroadcasts that transmission at the same frequency after amplifying it to a high level. If a mobile unit is in a location where it receives a signal at one frequency, from either the primary repeater or a secondary repeater, it need not be tuned differently to receive both signals. Spectral usage is that of the mobile unit alone, and use of the radio has been made the simplest for the operator. However, in areas where the mobile unit can receive signals from two repeaters, the possibility exists of simulcast distortion. This is distortion resulting from reception of identical frequency-modulated signals delayed in time by a significant amount. Such delay begins to be detectable at about 10 microseconds. Over a few tens of microseconds the distortion is annoying but does not destroy intelligibility. As the delay approaches and exceeds 100 microseconds, however, it becomes increasingly difficult or impossible to understand the composite signal. With a primary repeater alone, simulcast distortion occurs principally when a reflected signal interferes with a direct signal that has been weakened by obstructions or the like. If the difference in signal strength is more than about 10 dB, the stronger signal will be captured and the weaker signal will not interfere. The inherent delay in a repeater is typically of the order of 100 to 200 microseconds. For this reason, whenever a receiver receives signals from two transmitters, one of which is a same-frequency repeater, and those signals are within about 10 dB in relative signal strength, reducing the probability of capture, the detected signal will be greatly distorted.
There is an additional problem that can interfere with communication when using repeaters. If a repeater is not synchronized in frequency with another transmitter operating at nearly the same frequency, the possibility exists of beats between the different carriers. If such beats occur above about 50 Hz, they become audible and annoying. To minimize beats with present repeaters providing overlapping coverage, it is common to use transmitters with carrier frequencies related to very high-stability oscillators such as the output of rubidium-standard oscillators. Such oscillators can achieve frequency control that eliminates audible beats, but at considerable cost.
It is an object of the present invention to minimize beats between signals when using same-frequency repeaters.
It is further object of the present invention to eliminate simulcast distortion between signals from SFR's at a relatively low cost.
It is a further object of the present invention to prevent distortion resulting from the reception of two radio signals identical in content and delayed in time when using a same-frequency repeater.
Other objects will become apparent in the course of a detailed description of the invention.